Several systems are known by which the locations of points in space can be identified and digitized. If these points are disposed on an object, and the size and shape of the object are predetermined or are known, it is possible to calculate the position and orientation of the object from knowing the locations of these several points on the object. In advanced systems, the position and orientation of a plurality of objects can be determined both independently with respect to the three dimensional space in which each object resides, and with respect to the other object(s). It is even possible, by frequent sampling of the locations of at least two points on an object, to track movement of the object, both absolutely with respect to the three dimensional space and relative to the position and orientation, and even movement of other objects(s) in the same space.
In one embodiment of these known systems, a plurality of emitters of electromagnetic radiation, such as light of a given wavelength, are fixed to the surface of at least one object. If it is desired to know the absolute positions and orientations of the objects being determined, the radiation emitted by the emitters can be received by a plurality of cameras, or other receivers which are in a known position in the three dimensional space. If, on the other hand, it is only desired to determine the position and orientation of the several objects relative to each other, the cameras do not need to be in known or fixed positions relative to the three dimensional space in which they reside, although they must still be in known relationship to each other. The radiation from the emitters to the receivers form straight lines and the angles that these ray lines make with respect to each other or with respect to predetermined reference lines or planes, respectively, can be used to compute the location of each radiation emitter in space. The location of each radiating emitter can be digitized, and all of the determined locations can be used to calculate the position and the orientation of the object in space.
It is desirable to use computers to assist in the calculation of the geometric relationships which derive the locations of the emitters. Therefore, the output of the camera systems that are used is preferably fed to a digital computer for calculating the necessary angles and digitized point source locations, and for converting these to the position and orientation of the object on which the emitters reside. The geometric calculations, and the algorithms that control these calculations do not form a part of this invention.
The accuracy of determining the location of the emitters is in great measure a function of the stability of the emitting point. One type of emitter that has been used with considerable success is a light emitting diode, an LED. In actual practice, the best LEDs have been those that emit light in the infra red spectrum. Since the mensuration device being discussed here has great utility in hospital operating rooms, the use of "invisible" light beams has an added advantage of not distracting the surgeon from his work. Light, with wavelengths in the visible spectrum, is typically filtered out by the optical sensor(s) to reduce interference with the "invisible", or infrared, radiation.
Optical emitters are generally housed in an assembly which protects them from dirt and other external influences. Part of this protection is commonly provided by a relatively clear crown over the emitter and a support (header) affixed to the object on which the emitter rests. The crown is normally mounted to the support, and the emitter (the LED chip) is generally centrally located on the support under the crown. Radiation is emitted from the emitter in substantially all hemispherical directions; from a direction normal to the surface on which the emitter is affixed to a direction substantially parallel or tangent to that surface. So long as the line of sight between the emitter and the camera approaches the tangent, the crown tends to make the optical location of the centroid of the emitter appear to move due to refraction of the radiation. Therefore, the camera "sees" the emitter at a place where it is not. While it is true that the dislocation of the apparent location of the emitter from the real location of the emitter is small, the resultant system inaccuracy can be very substantial. This is of particular importance when very accurate determinations of the positions and orientations of objects in space, such as surgical instruments, is being determined. It is not possible to be too accurate in determining the real location of the emitters so as to be able to very accurately determine the true position and orientation of the body on which the emitters reside. Put another way, the optical center of the emitter must appear to the camera to be in the same relative location regardless of the angle through which the emitter is viewed.